1. Field of the Invention
The present invention relates to an apparatus for processing an image signal obtained from a sensor such as a CCD sensor and, more particularly, to a processing apparatus suitable for reducing noise of the image signal.
2. Related Background Art
Noise reduction utilizing spatial correlation or time-correlation of a video signal is performed in an existing video signal recording/reproducing apparatus to improve as S/N ratio during recording or reproduction. FIG. 8 shows a typical cyclic noise reduction circuit utilizing a field memory. The noise reduction circuit includes a multiplier 110 for multiplying an input with a coefficient (1-K) where K is 0.ltoreq.K&lt;1, an adder 111, a multiplier 112 for multiplying an input with a coefficient K, and a field memory 113 serving as a delay element for a one-field period. The transfer function of this digital filter is given as follows: EQU H(Z.sub.f)=(1-K)/(1-KZ.sub.f.sup.-1)
This transfer function exhibits low-frequency filtering characteristics as a function of time (i.e., between fields). In the equation, Z.sub.f.sup.-1 represents a unit delay operator corresponding to the one-field delay. This circuit reduces noise in a still image portion.
When the memory 113 in the arrangement of FIG. 8 is replaced with a 1H (H is a horizontal scanning period) line memory, a noise reduction circuit utilizing correlation in the vertical direction can be arranged. .
In a noise reduction circuit utilizing time-correlation, an after-image is undesirably formed when an object in the image is moving. In a noise reduction circuit utilizing spatial correction, an edge of the image becomes unclear.
A system has recently been proposed to detect a moving portion of an image and its edge to properly change the coefficient K. However, this system is not yet satisfactory in practical applications. In this adaptive control system, image quality is adversely influenced by unclearness of the edge and by an after-image. These adverse influences are often greater than the effect of improving image quality by noise reduction.
When an image signal level is low, an automatic gain control (AGC) circuit is normally arranged in a processing circuit for processing such image signal to amplify the level of the image signal.
When an image pickup element such as a CCD is used as an image signal source, noise of some level is generated at an output unit, e.g., an output amplifier, regardless of the signal level. When the gain of the output signal is controlled by the AGC circuit, the noise level is increased at the same rate as the signal level. When an S/N ratio is low as in photography with a low illuminance, image quality is greatly degraded if the gain is excessively increased by the AGC circuit. Therefore, a great gain increase as indicated by a characteristic curve (c) in FIG. 13 cannot be provided.
FIG. 13 is a graph showing characteristic curves in exposure control and gain control. The illuminance (1.times.) is plotted along the abscissa in FIG. 13, and the controlled signal level is plotted along the ordinate in FIG. 13. The characteristic curve (a) in FIG. 13 shows characteristics under exposure control. The amount of incident light can be controlled to be an optimal signal level upon exposure control such as opening/closing of an aperture for an optical image having an illuminance of 10 1.times. or more. In this case, the illuminance of 10 1.times. corresponds to an exposure control limit in a fully open aperture state. The characteristic curve (b) in FIG. 13 is a composite characteristic curve when exposure control and gain control are performed. Gain control aims at compensating for a delay of exposure control when the illuminance is 10 1.times. or more. Level control is performed such that a signal level comes close to the optimal signal level at an illuminance of 10 1.times. or less.
The above problem is not limited to the video camera described above but is also experienced when random noise is generated together with an image signal. For example, the same problem as described above is posed, e.g., when an image signal is transferred through a system which generates random noise.